We want to relate the diversity of amino acid sequences among H1 histone subtypes and H1 variants (e.g. H1o and H1t) to the role of H1 in the structure of the chromosome. In particular we are interested in differences among H1 histones in their ability to condense chromatin (and related models) into higher order folding as a function of salt concentration. Complexes of individual kinds of H1 and chromatin fragments will be studied by circular dichroism, hydrodynamics, nuclease digestion and electron microscopy. Structural diversity among H1 subtypes and variants will be measured by amino acid sequencing and by immunological techniques. The kinetics and location of H1o will be studied throughout an extended period as cultured cells stop dividing and maintain themselves in a non-dividing state. When cells stop dividing, the non histones HMG1 and HMG2 are lost from the cell if the cells commit themselves to differentiation, and we want to look at the mechanism of this change in terms of proteolytic degradation or modification of the HMG's. These studies are aimed at showing if H1 histones play specific roles in controlling the dynamics of chromatin in cell division and differentiation, and so are related to problems of normal and abnormal growth.